Merge branch '534_federation_targets_reachability' into 'develop'

[#534] Unreachable federation targets retirement

Closes #534

See merge request pleroma/pleroma!703

priv/repo/migrations/20190123125546_create_instances.exs

@@ -0,0 +1,15 @@
+defmodule Pleroma.Repo.Migrations.CreateInstances do
+  use Ecto.Migration
+
+  def change do
+    create table(:instances) do
+      add :host, :string
+      add :unreachable_since, :naive_datetime
+
+      timestamps()
+    end
+
+    create unique_index(:instances, [:host])
+    create index(:instances, [:unreachable_since])
+  end
+end